gh-161518: Reject a non-bytes encoder result unless it is the ASCII fast path

Lib/test/test_io/test_textio.py

@@ -1097,6 +1097,31 @@ def test_illegal_encoder(self):
             t = self.TextIOWrapper(self.BytesIO(b'foo'), encoding="rot13")
         self.assertRaises(TypeError, t.write, 'bar')
 
+    def test_encoder_returns_str(self):
+        # gh-161518: an encoder returning its non-ASCII input str must be
+        # rejected, not have its internal storage written as bytes.
+        class Encoder(codecs.IncrementalEncoder):
+            def encode(self, input, final=False):
+                return input
+
+        info = codecs.CodecInfo(
+            None, None, incrementalencoder=Encoder,
+            incrementaldecoder=codecs.getincrementaldecoder('utf-8'),
+            name='test_str_encoder')
+        search = lambda name: info if name == 'test_str_encoder' else None
+        codecs.register(search)
+        self.addCleanup(codecs.unregister, search)
+
+        t = self.TextIOWrapper(self.BytesIO(), encoding='test_str_encoder')
+        self.assertRaises(TypeError, t.write, 'caf\xe9')
+
+        # The ASCII fast path still works.
+        raw = self.BytesIO()
+        t = self.TextIOWrapper(raw, encoding='utf-8')
+        t.write('cafe')
+        t.flush()
+        self.assertEqual(raw.getvalue(), b'cafe')
+
     def test_illegal_decoder(self):
         # Issue #17106
         # Bypass the early encoding check added in issue 20404

Misc/NEWS.d/next/Library/2026-11-06-21-50-00.gh-issue-161518.Tx9Kq2.rst

@@ -0,0 +1,2 @@
+:class:`io.TextIOWrapper` now raises :exc:`TypeError`, instead of writing
+corrupt output, when an encoder returns its non-ASCII input string.

Modules/_io/textio.c

@@ -1831,7 +1831,7 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
     Py_DECREF(text);
     if (b == NULL)
         return NULL;
-    if (b != text && !PyBytes_Check(b)) {
+    if (!PyBytes_Check(b) && !(b == text && PyUnicode_IS_ASCII(text))) {
         PyErr_Format(PyExc_TypeError,
                      "encoder should return a bytes object, not '%.200s'",
                      Py_TYPE(b)->tp_name);